In some air-conditioning apparatuses, like a multi-air-conditioner for a building, a heat source unit (outdoor unit) is placed outside a structure, and an indoor unit is placed in the indoors of the structure. Refrigerant that circulates through a refrigerant circuit of such air-conditioning apparatuses rejects heat to (removes heat from) air supplied to a heat exchanger of the indoor unit to thereby heat or cool the air. Then, the heated or cooled air is sent to an air-conditioned space to perform heating or cooling. As refrigerants used in such air-conditioning apparatuses, for example, hydrofluorocarbon (HFC)-based refrigerants are frequently used. Air-conditioning apparatuses using natural refrigerants such as carbon dioxide (CO2) have been also proposed.
An air-conditioning apparatus with a different configuration represented by a chiller system has been also proposed (for example, Patent Literature 1). In the technique described in Patent Literature 1, cooling energy or heating energy is generated in a heat source unit placed outdoors, a heat medium such as water or antifreeze liquid is heated or cooled by a heat exchanger placed inside an outdoor unit, and this is conveyed to an indoor unit such as a fan coil unit or a panel heater placed in an air-conditioned area to thereby execute cooling or heating.
There has been also proposed an air-conditioning apparatus in which a water pipe through which heated water flows, and a water pipe through which cooled water flows are individually connected between a heat source unit and an indoor unit (see, for example, Patent Literature 2). The technique described in Patent Literature 2 switches connections so that in a heating operation the water pipe through which heated water flows and the indoor unit are connected, and in a cooling operation the water pipe through which cooled water flows and the indoor unit are connected, thereby allowing cooling or heating to be freely selected.
There has been also proposed an air-conditioning apparatus configured so that a heat exchange unit provided with a heat exchanger for exchanging heat between a primary refrigerant and a secondary refrigerant is placed near an indoor unit, and the secondary refrigerant is conveyed to the indoor unit from the heat exchange unit (see, for example, Patent Literature 3).
There is also an air-conditioning apparatus configured so that an outdoor unit and a branch unit having a heat exchanger are connected by two pipes, and a secondary refrigerant is conveyed to an indoor unit (see, for example, Patent Literature 4).